The attachment of components to printed circuit boards (PCBs) produces printed circuit board assemblies (PCBAs), which can be used for motherboards in computers systems, peripheral devices, hand held personal electronic devices, or any electronic device that supports an integrated circuit. A PCB may be a laminated board made of an insulating material such as plastic which contains several layers of metal such as copper separated by insulating material or it may be made of a flexible Mylar backing having plated copper or aluminum traces and interconnects. The metal may function to establish electrical connections between parts mounted on the board, conduct heat, or provide a shield or reference voltage.
One increasingly popular component of PCBAs is a quad flat pack no-lead (QFN) chip. QFNs. A QFN is an electronic component encapsulated in plastic, ceramic, or some other insulating material. A QFN contains rows of IO pads, areas in which bare metal is exposed, on each of its four sides (hence, the “quad” in QFN) for electrical connectivity with the PCB. The QFN also typically contains a thermal pad underneath, an exposed area of metal for conducting heat away from the package.
A QFN may be light, present a small footprint, and feature good thermal and electrical conductivity. The small footprint conserves space on the PCB, which can be scarce due to the industry trend to miniaturize products and add additional functions.
Good thermal conductivity helps to maintain the QFN and the point of connection at an acceptable temperature, thus preserving the useful life and reliability of the chip. A QFN can be attached to a PCB by soldering it directly to a PCB. Due to the small size of the package and the close proximity of the I/O pads, precise positioning and proper solder techniques are required for volume assembly.
QFNs may prove more difficult to attach to PCBs than components with leads, which may be attached to a PCB by soldering the lead to the PCB. Soldering together two flat planes, the QFN thermal pad and the PCB, may be more difficult than soldering a lead from a leaded component to the PCB.
To solder the QFN to the PCB, solder paste, which may contain solder and flux chemicals, can be applied to the surface of the PCB at appropriate regions. The solder paste can be applied to the PCB surface by extrusion through a stencil. The solder paste can be placed on the stencil and forced through the apertures of the stencil by pressing with a squeegee.
After the application of solder paste, the QFN can be positioned on the PCB, and the assembly placed into an oven or series of ovens and heated. The heating can evaporate the flux chemicals and other solvents and cause the solder to melt, leading to wetting and wicking. A solder mask can also be placed on the PCB to control the solder paste during heating. The solder mask defines openings on the outer layers of the PCB and exposes the copper features of the PCB. The solder mask helps to prevent the liquid solder from flowing away from the desired areas of solder application. As the number of I/O pads increases and the package size is decreased, the risk of solder flowing between multiple I/O pads increases.
The solder mask is placed over the PCB, and solder paste is applied to areas of the PCB to which the QFN is to be attached that are not protected with a solder mask. Due to the small spaces between the I/O pads and the flat surfaces in close proximity, a capillary effect may cause the solder to flow across the solder mask to inadvertently couple the I/O pads. This difficulty is not evident by inspection because the connections may be under the package itself.
To prepare for attachment of a QFN, the area of the PCB on which the QFN will rest may be fitted with I/O pads and a thermal pad, regions for contact with the QFN I/O pads and QFN thermal pad. The pads may consist of copper or another metal. When the QFN is attached to the PCB, the QFN pads rest on the corresponding PCB pads and are connected with solder.
The PCB pads may be slightly larger than the QFN pads to provide tolerance for imperfect placement. The QFN I/O pads may be soldered to the PCB I/O pads to provide an electrical connection between the PCB and the QFN. The thermal pad of the QFN may be soldered to the PCB thermal pad to provide thermal conductivity and a mechanical connection and can also provide an electrical connection.
The direct soldering of bare metal areas of the QFN to the surface of the PCB may provide for good electrical and thermal conductivity as well as a good mechanical connection. To conduct away heat transferred from the QFN thermal pad to the PCB thermal pad, the PCB thermal pad region may contain vias.
Generally, solder does not cover the entire PCB thermal pad. Instead, smaller regions of solder, called solder pads, can be deposited on the PCB thermal pad. The amount of coverage may be expressed as a percentage. For example, 50% coverage indicates that half the area of the thermal pad is covered with solder.
The amount of solder placed on the PCB thermal pad is critical to the attachment process. When too much solder is placed on the PCB thermal pad, the QFN may actually float on top of the solder. The heating process may create a ball of solder in the middle of the QFN on which the QFN floats.
The QFN may begin to turn, depending on how the QFN was placed on the PCB and on other factors such as air movement and vibration. The movement of the QFN may create shorts in the QFN I/O pads. Movement of the QFN I/O pads may cause solder to smear from one QFN I/O pad to another. Since modern QFN packages may have multiple rows of I/O pads that are flush with the bottom of the package, the effects of solder smearing and the capillary effect can dramatically reduce the manufacturing yield.
Further, the floating of the QFN on the excess solder in the thermal pad region can interfere with the forming of solder joints in the IO pad regions. On the other hand, too little solder can cause a poor or non-existent connection between the QFN and the PCB.
In addition to the total amount of solder, the size of the individual solder pads affects the attachment process. A solder pad with minimum diameter smaller than the width of a stencil through which the solder pad is extruded may not deposit out of the stencil in the proper shape.
Thus, a need still remains for an integrated circuit package system with package stand-off, that allows a finished assembly yield improvement. In view of the constant demand for low cost electronic assemblies that provide additional functions in restricted space, it is increasingly critical that answers be found to these problems. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations and the diminishing opportunities for meaningful product differentiation in the marketplace, it is critical that answers be found for these problems. Additionally, the need to reduce costs, improve efficiencies and performance, and meet competitive pressures adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.